1. Field of the Invention
The present invention relates to an information recording medium having a recording layer of a cyanine dye for recording information by means of a laser beam having a high energy density and further relates to a novel cyanine dye employable for the information recording medium.
2. Description of Prior Art
An optical disc capable of recording information only once is referred to "DRAW" (Direct Read After Write), and studies on the optical disc have been made.
The optical disc of DRAW-type basically comprises a disc-shaped substrate made of a plastic material or a glass material and a recording layer made of a metal such as Bi, Sn, In and Te, a semi-metal or a dye such as a cyanine dye, a metal complex dye or a quinone dye, which is provided on the substrate. On the surface of the substrate where the recording layer is to be provided, an intermediate layer may be provided to improve surface smoothness of the substrate and to enhance adhesion between the substrate and the recording layer or sensitivity of the resulting optical disc. Writing (i.e., recording) of information on the optical disc can be carried out by irradiating the disc with a laser beam. The irradiated area of the recording layer of the optical disc absorbs energy of the laser beam and rise of temperature locally occurs, and as a result, a chemical or physical change (e.g., formation of pits) is caused to alter (or change) optical characteristics of the recording layer in the irradiated area, whereby information is recorded on the recording layer. Reading (i.e., reproduction) of information from the optical disc is also carried out by irradiating the disc with a laser beam, that is, the recorded information is reproduced by detecting a reflected light or a transmitted light corresponding to the change in the optical characteristics of the recording layer.
As a material (i.e., recording material) for forming the recording layer of the information recording medium, metals and dyes (e.g., cyanine dye) are generally known, as described above. An information recording medium using a dye as a recording material has a merit in the characteristics of an information recording medium, for example, the information recording medium has a higher sensitivity than the information recording medium using a metal as a recording material. In addition, the recording medium using a dye has such an advantageous feature in productivity that the recording layer can be easily formed on the substrate by a conventional coating method. For these reasons, dyes employable for such optical disc have been developed and put into practical use. The properties required for the dyes in application to the optical disc are for example as follows: property of absorbing a light in the wavelength region of 700 to 900 nm, that is a wavelength region of a semiconductor laser; relatively large absorption factor; and high reflectance. Recently, studies on enlarging of the wavelength region of a semiconductor laser or improvement and development of an optical disc drive have been made, and in accordance therewith, increase in number of dyes employable for the optical disc and variation thereof are expected. Therefore, it is desired that various kinds of dyes employable for the optical disc are developed.
The recording layers made of the above-mentioned dyes have such problem in optical characteristics that the layer generally has low reflectance and low C/N of the reproduced signals. Further, such dye recording layer easily deteriorates with lapse of time owing to irradiation with a light.
Japanese Patent Provisional Publication No. 64(1989)-40382 discloses an optical disc having a recording layer made of a cyanine dye having benzoindolenine structure as a dye recording layer improved in the above-mentioned reflectance and C/N. An information recording medium having such dye recording layer is relatively high in C/N, but is unsatisfactory in the reflectance and weathering resistance.
For enhancing a reflectance of the information recording medium, a method of further providing a reflecting layer on the dye recording layer is generally made, and an example of the information recording medium having a reflecting layer on the dye recording layer is described in "Nikkei Electronics", p. 107, Jan. 23, 1989. In the publication, although dyes used for the recording layer of the information recording medium are not apparent, recording of information on the recording layer is described as follows. That is, when the dye recording layer absorbs a laser beam to melt the dye of the layer, the plastic substrate is heated to protrude on the dye recording layer side to form pits, so as to record information on the recording layer. The reflecting layer provided in this information recording medium is a layer of Au-deposited film. According to the study of the present inventors, in the case that the above-mentioned cyanine dye having benzoindolenine structure is used for the dye recording layer, an optical disc having a high C/N and improved reflectance is obtained.
In the optical disc of DRAW-type, high-density recording of CD format signals (i.e., CD-DRAW) is necessarily made at a low linear speed of 1.2 to 1.4 m/sec, and the recorded signals are required to be reproduced using commercially available CD players. In the case of reproducing the recorded signals using the CD players, the optical disc is desired to have a reflectance of not less than 70%. However, even if CD format signals are recorded on the optical disc having a recording layer made of a cyanine dye having the above-mentioned benzoindolenine structure and a reflecting layer on the recording layer, some of the commercially available CD players cannot reproduce the recorded signals.
Accordingly, it is desired to produce optical discs having a markedly high reflectance (approx. 80%) which are employable as the above-mentioned CD-DRAW type discs.